Scroll compressors are used in refrigeration systems such as refrigerators, freezers and air conditioners. As shown in FIGS. 1a-1c, a scroll compressor is always provided with a stationary scroll member 14 and an orbiting scroll member 18 which rotates around the center of the stationary scroll member 14. In other words, the orbiting scroll member 18 is orbiting round the center of the stationary scroll member 14. During operation, working fluid 3 to be compressed is guided to enter a space 19 enclosed by the stationary scroll member 14 and the orbiting scroll member 18 (see FIG. 1a), then the enclosed working fluid 3 is progressively compressed by the orbiting motion of the orbiting scroll member 18 and finally discharged from the scroll compressor by way of the discharge port 19g (see FIGS. 1b and 1c).
FIG. 2 is a cross sectional view showing the whole construction of a conventional scroll compressor 12 which has been disclosed in detail in U.S. Pat. No. 4,365,941.
When the orbiting scroll member 18 is driven to rotate about the axis of the stationary scroll member 14, the orbiting scroll member 18 is subjected to an axial force "F" shown in FIG. 2, which tends to push the orbiting scroll member 18 away from the stationary scroll member 14. If the axial force "F" cannot be overcome, the clearance between the orbiting scroll member 18 and the stationary scroll member 14 will be enlarged, and working fluid being compressed will leak through the enlarged clearance. Thus, the volumetric efficiency of the scroll compressor will be reduced.
Two ways have been proposed to overcome the tip surface leakage problem occurred in conventional scroll compressors.
First, U.S. Pat. No. 4,564,343 discloses a resilient sealing element. As shown in FIGS. 3 and 4, two resilient sealing elements 8 are embedded respectively into tip surfaces 7a, 7b of the scroll wraps 18b, 14b of the orbiting scroll member 18 and the stationary scroll member 14. By this arrangement, clearance formed between the tip surface 7a of the scroll wrap 18b and the end plate surface 14s of the stationary scroll member 14 as well as clearance formed between the tip surface 7b of the scroll wrap 14b of the stationary scroll member 14 and the end plate surface 18s of the orbiting scroll member 18 will be blocked, and leakage of working fluid to be compressed will be reduced.
However, resilient sealing elements 8 are embedded in the tip surfaces 7a and 7b of the scrolls 18b and 14b, and the width "w" of the resilient sealing elements 8 should be smaller than the thickness "t" of both scrolls wrap 14b and 18b. Thus, working fluid will still leak out along the peripheral direction "i" of the scroll. In addition, resilient sealing elements 8 will inevitably wear out, and leakage amount of working fluid will increase time by time.
FIG. 5 shows the second way used in U.S. Pat. No. 4,365,941 to overcome leakage problem. FIG. 5 is a fragmentary view of a scroll compressor with some minor modifications to the stationary scroll member 14 thereof. As shown in FIG. 5, two small through holes 18d are formed in the orbiting scroll member 18. These two holes 18d communicate the backpressure chamber 31 with the compression chambers 19a and 19b, thus the backpressure chamber 31 is maintained at the same pressure as that of compression chamber 19a and 19b. By this arrangement, a resultant forge "P" pushing the orbiting scroll member 18 upward at the center of geometry thereof will be induced, and the force "F" pushing the orbiting scroll member 18 away from the stationary scroll member 14 will be counteracted. Nevertheless, the resultant force "P" should be larger than the resultant force "F" by at least an amount that the orbiting scroll member 18 will not wobble due to a lateral resultant force "R" exerting on the scroll wrap 18b of the orbiting scroll member 18. For this reason, a large resultant force "P" is required, and thus the orbiting scroll member 18 suffers from a great frictional force. Consequently, mechanical efficiency of scroll compressors will thus be reduced. In addition, an undesirable mixing of the lubricant and the working fluid will inevitably arise due to that the lubricant enters the compression chambers by way of the holes 18d, and the volumetric efficiency of scroll compressors will be diminished.